Spherical silica particles are used in a variety of purposes. For example, Patent Literature 1 (Japanese Unexamined Patent Application Publication No. H07(1995)-140472) describes the production of spacer particles for liquid crystal cells by heating treatment (at from 100 to 1000° C.) of particles obtained from the hydrolysis and poly-condensation of an organic silicon compound. In addition, Patent Literature 2 (Japanese Unexamined Patent Application Publication No. H11(1999)-228699) and Patent Literature 3 (Japanese Unexamined Patent Application Publication No. H11(1999)-228698) report that silica particles obtained from the hydrolysis and condensation of an organic silicon compound are calcined at high temperature and the calcined silica particles are used for an in-plane spacer or a sealing spacer of liquid crystal displays.
In addition, calcined silica particles are added as a filling agent or filler in resin compositions. For example, it is known that calcined silica particles are added as a filling agent in curable resin compositions used as sealing materials of semiconductor devices and dental materials. Calcined silica particles are required to have low hygroscopicity, a narrow particle size distribution with a uniform particle size, high dispersibility, and the like. For example, Patent Literature 4 (Japanese Unexamined Patent Application Publication No. 2012-142438) discloses a paste used for mounting semiconductor elements, which includes polyorganosiloxane particles having a mean particle size from 0.5 to 30 μm and a coefficient of particle size variation (CV value) of 3% or less and resin.
In addition, Patent Literature 5 (Japanese Unexamined Patent Application Publication No. S62(1987)-96313) and Patent Literature 6 (Japanese Unexamined Patent Application Publication No. H01(1989)-234319) disclose that silica particles obtained by condensation of a hydrate are uniform in particle size and therefore suitable as a filler or the like.
Patent Literature 7 (Japanese Unexamined Patent Application Publication No. 2003-176121) discloses calcined silica particles having a mean particle size in a predetermined range and thus having a small standard deviation of the mean particle size, containing few agglomerates thereof, and having low hygroscopicity. Specifically, silica particles are made by hydrolyzing and condensing a hydrolyzable silicon compound in an organic solvent including water and a catalyst. After these silica particles are spray dried, the silica particles are calcined in a range of 1000 to 1200° C. In this case, since the silica particles are dried in an agglomeration state, when the silica particles are calcined, they will be progressively fused to form agglomerates, monodispersed paericles are not obtained. Owing to this, the silica particles, after being spray dried, are pulverized using a pulverizer such as a hammer mill (a pulverization step), resulting in pulverizing the agglomerates and suppressing the agglomeration during the calcination of the silica particles.
In addition, Patent Literature 8 (Japanese Unexamined Patent Application Publication No. 2011-245362) discloses a jet mill in which a high pressure gas is introduced in a truly circular chamber to concentrically generate a swirling flow running along an inner wall surface of the chamber, whereby raw material particles are caused to collide with each other to be pulverized and powdered.